D. After the mechanic took a few minutes to vacillate the engine, Irina's car ran perfectly.

If you quickly made up your mind that A was the right answer, good thinking! Sentence B uses "vacillate" as a noun, and since we know it's a verb, you can toss that option out. Any number of words could be plugged into C to replace "vacillate," whose meaning just doesn't agree with its use in the sentence. Some possibilities would be "talk," "speak," "chat," and "help." Since you can't "vacillate" an engine (or anything else, for that matter), D is out of the running too. Those indecisive voters in A, though, illustrate the meaning of "vacillate" perfectly, making sentence A the right choice.